My husband and I have a small terrarium that we use for succulents. We purchased it from the local gardening store for only $30. We were happy to get a small container and plant three or four different types of succulents. One of the plants had been in the ground for only a few days, and after we water it up we noticed that the soil was already damp with moisture. I read somewhere that if your terrarium is too wet there can be problems with root growth.
Our terrarium is only two feet long, so it’s really small. We don’t want to worry about the drainage issues that some bigger terrariums have. But I did read somewhere that a larger terrarium with a gravel layer serves the purpose of giving the plant roots the proper drainage they need. The gravel layer serves as a built in water reservoir that keeps the soil moist. But that is just a small part of the drainage requirements.
For example, my indoor plantings are watered only twice a day. But our large terrarium with a gravel bed needs to be watered every day. We use a high humidity solution because we want the moss to grow faster and more succulent. I read that activated charcoal added moisture to the air, but we don’t have a lot of clay so I’m not sure if that’s what activated charcoal does.
Watering is something we do quite often. I have used a hydrophobic weed barrier in my flowerbeds, along with slow releasing organic fertilizer. I also let the soil dry out between waterings. My indoor plant succulents have flourished under these conditions. So it seems that the soil moisture problem is caused by my own lack of watering.
Now, let’s look at the other common problems of closed terrariums. They are usually over-watered, have poor air circulation or too much light. In my case, insufficient lighting means that most of my indoor plants don’t get their needed dose of sunlight. They also need to be given adequate nutrients and water.
I haven’t found any evidence that activated charcoal alone will solve all of these problems. That would be a big claim to make, but I’ve had good results with some of my terrarium plants growing under low light conditions. So it appears that the lack of sunlight isn’t the only reason they aren’t thriving. They may just need to be given additional light or extra nutrients. That could easily be accomplished by making the terrariums larger, opening them up to better air circulation, adding some sand to the soil, etc.
My biggest issue with terrariums that are too wet is that they encourage the growth of certain undesirable fungus species. Fungi will sometimes grow where there is no oxygen, so I believe this is why some succulents do poorly. I’ve also seen some reports online from people saying that certain types of fungus, such as Stachybotrys, do well in water. I’m not sure if that’s true, but it seems like it would make sense for those looking to buy their plants from an online source.
If you’re looking for a way to avoid this problem, look for a brand that offers a variety of substrates. I prefer to use gravel, but I think that some clay mixes are fine. If you’re only concerned about water and nutrients, go with one of the simpler gravels, such as stuff from your local garden store. On the other hand, if you’re looking for some shade and air, go with one of the more heavy duty sand mixes, such as those offered by Reefquest. You might also be concerned about keeping your plants healthy. It’s always best to check your soil layers, and introduce some additional nutrients to the mix as needed, rather than to cover up the problem with a water soak.